1. Field of the Invention
This invention relates to the manufacture of cement clinker in long rotary kilns. In particular, this invention is directed to the manufacture of cement clinker in conventional long wet or dry rotary kilns wherein volatile fuel components are added directly into the calcining zone of such rotary kilns.
2. State of the Art
Cement clinker may be produced by heating calcareous material with an argillaceous material or other forms of silica, alumina, and iron oxide which may additionally include minor amounts of materials indigenous to these raw materials, at temperatures on the order of about 2300.degree.-2900.degree. F. to bring about the chemical reactions necessary to convert the ingredients to calcium silicates, i.e., cement clinker. The literature is replete with processes by which the calcining and clinkering of cement ingredients can be accomplished. For example, cement raw materials may be mixed with solid fuel and burned with air on a grate to provide a final clinker (e.g., U.S. Pat. Nos. 2,090,363 and 3,135,618) or cement raw materials mixed with fuel may be sintered with air on a grate or the like and the sintered intermediate then clinkered in a rotary kiln, blast furnace or the like (e.g., U.S. Pat. Nos. 1,746,944 and 1,904,699). In still other alternatives, cement raw materials and fuel may be formed into shapes such as pellets, briquets and the like and then burned and clinkered (e.g., U.S. Pat. Nos. 274,288, 1,132,527, 2,904,455, 2,991,187 and 3,127,455).
In one widely used commercial process to which this invention is directed, calcining and clinkering of cement raw materials is accomplished by passing finely divided raw materials through a rotating inclined rotary vessel termed a conventional long wet or dry rotary kiln. In this process, the requisite temperatures are achieved by burning fuel such as gas, fuel oil, powdered coal, coke or the like, singularly or in combination in the gaseous atmosphere of the kiln, with the gases moving countercurrent to the solids through the kiln. Inasmuch as high temperatures are required for the process, fuel costs constitute a significant factor in the ultimate cost of the product. In particular, it is art recognized that the most significant factor in overall fuel costs for the production of cement clinker is the highly endothermic calcining step of converting calcium carbonate to calcium oxide with the concomitant generation of carbon dioxide. This step alone accounts for over 70% of the theoretical energy requirement of a typical dry process.
Methods of reducing fuel costs, especially the fuel costs for the calcining step, accordingly have been and still remain a major focus for the industry. One recent method for realizing substantial fuel savings is the use of a precalciner. Precalciners contain a special chamber to allow combustion of up to 60% of the total process fuel in suspension with preheated raw materials from the third stage of a conventional suspension preheater system to rapidly (typically 1-2 seconds) calcine about 90% percent of the calcium carbonate to calcium oxide.
In particular, in the precalciner, the raw materials are intimately suspended in hot air along with finely divided fuel components. The hot air initially employed in the precalciner is typically "recovered heat" derived from parts of the cement making process, e.g., the clinker cooler, etc. The fuel components are employed to generate additional heat so as to calcine approximately 90% of the calcium carbonate to calcium oxide. Because of the intimate contact between the suspended particles and the burning fuel, excellent thermal efficiency is achieved resulting in substantial fuel savings (among other benefits) as compared to the conventional long wet or dry rotary kilns. Because all of the calcium carbonate has not been calcined, a final calcining zone is still required in the kiln. However, this zone is substantially reduced as compared to the calcining zones of conventional long wet or dry rotary kilns. See, Garrett, Rock Products, "Precalciners Today--A Review", pp. 39 et seq., July 1985, for a detailed description of precalciners.
Another method of improving thermal efficiency and reducing fuel costs is disclosed by Garrett, U.S. Pat. No. 4,022,629. This reference discloses the use of low volatile, high ignition temperature, solid fuel components which can be added to the kiln either with the feed at the feed-end of the kiln or at any point in the process up to and including the calcining zone to provide for combustion predominantly within the calcining zone. Because the solid fuel particles are in the interior and/or on the surface of the bed itself and in contact with the cement raw materials, heat from this fuel is transferred by conduction, convection and radiation which results in enhanced thermal efficiency as compared to the thermal efficiency achieved by heating the feed with hot gases passing above the feedbed.
Still another method of reducing fuel costs is disclosed by Benoit et al., U.S. Pat. No. 4,850,290. The process disclosed by this reference employs combustible hazardous wastes including volatile hazardous wastes which have been containerized so as to render the wastes non-volatile during the addition period and accordingly, safe to handle. The entire containerized waste module is added directly into the calcining zone of the kiln via a drop tube originating from a port at an appropriate location on the kiln as disclosed in this reference. Upon addition of the containerized waste module to the kiln, the module is embedded in the feed and eventually combusts. Because of the very high temperatures employed in this portion of the kiln, complete combustion is ensured and the combustion products (e.g., primarily water, carbon dioxide and oxides of metal found in the container) are non-hazardous. Moreover, because suitable hazardous wastes for this process are readily available with little or no costs (indeed it is contemplated that there could be negative costs for these fuel elements), the use of such waste results in large fuel cost savings.
However, the charging of volatile fuel components into the feed end of a long wet or dry rotary kiln along with the to-be-processed mineral materials or into the pre-heat zone is expected to result in unacceptable emissions of hydrocarbons due to the release of the volatile components of the fuel in a region of the kiln where the temperatures are too low for ignition and to result in poor energy recovery due to the ignition of the remaining fuel prior to the calcining zone where the energy can be utilized.
On the other hand, cement kilns, equipped with air suspension preheaters, lepol grate preheaters, and the like, have added high volatile fuel components to the feed end of such preheater kilns. The high volatile fuel components rapidly burn in the early part of the calcining zone and generate heat which is primarily transferred to the gas exiting the kiln. This heated gas enters the gas inlet of the air suspension preheater, lepol grate preheater, and the like as a cheap and efficient means to obtained preheated inlet gas for rapid heating and calcining of feed materials suspended in the gases. See, for instance, Naredi., Refra Symposium '82, pages 21-42 as well as Ogawa, U.S. Pat. No. 4,295,825. This practice is usually limited to 10% of the total process fuel energy since the capacity of the process to utilize fuel burned at this point is limited because additional energy input beyond this results in increasing the process exit gas temperature. In view of the above, it is apparent that these prior art methods using preheaters are for the purpose of providing the heat necessary to heat the gas for use in the preheater device and accordingly, has significantly different thermodynamic considerations as compared to heating solid feed materials in the calcining zone.
In still another prior art method, high volatile solid fuels have been used in suspension precalciners as a cheap source of fuel to effect calcination. In particular, precalciners have employed tires, cut into pieces sufficiently small to be dispersed so as to permit their use as fuel in precalciners. When so used, any portion of the fuel not burned in the precalciner chamber will fall into the feed end of the kiln where the remainder of the fuel is combusted which in turn heats the gas exiting the kiln. Because of the small size of the tire pieces and because of the high air temperature employed, sufficiently complete oxidation is achieved so that the exiting gas meets current EPA guidelines.
The use of volatile, waste derived, or other low quality fuels that could be obtained at minimal or no cost that are by their nature not suitable to be prepared in a manner that would allow them to be burned dispersed in a flame or, due to their low heating value, have an adverse effect on flame temperature, would impart a substantial fuel cost savings if they could be substituted for the primary fossil fuels conventionally employed in long wet or dry rotary kilns. Moreover, if these fuels could be used to generate heat directly in the calcining zone, the energy content of these fuels could be more efficiently used and a substantial portion of the process energy could be supplied by these low cost fuels. Additionally, in view of the fact that the majority of the heat required to manufacture cement clinker via a conventional long wet or dry rotary kiln occurs in the calcining zone, the use of high volatile fuels in the calcining zone would be particularly advantageous. However, in spite of the economic benefits, use of such fuels in the calcining zone of a conventional long wet or dry kiln without first containerizing these fuels so as to render them non-volatile was thought to be impractical for several reasons. Firstly, all of the fuel added to the calcining zone must be substantially burned (particularly residual carbon) before it enters the clinkering zone. In particular, large amounts of carbon entering the clinkering zone create reducing conditions which change and degrade the quality of clinker in terms of strength, color, etc. Secondly, if the volatile fuel added to the calcining zone volatilizes and burns too quickly, then the fuel will overwhelm the available oxygen in the kiln and result in unacceptable emissions of hydrocarbons, carbon monoxide and the like in the gases exiting the kiln. Additionally, it was heretofore believed that it was not possible to add high volatile temperature fuel directly into an area of the calcining zone of a conventional long wet or dry rotary kiln where the energy could be most efficiently used because of the high temperatures (significantly greater than 2000.degree. F.). Further, recently proposed emission standards for waste combustion, including municipal and hazardous wastes, that are so strict (as compared to those for fossil fuel combustion) that it was believed that any such use of waste fuels in a long kiln by other than dispersion in the flame in the sintering zone would result in non-compliant emissions.
In view of the above, it is one object of this invention to provide a process for the manufacture of cement clinker in a conventional long wet or dry rotary kiln characterized, inter alia, by the combustion of volatile fuel elements directly in the calcining zone of the kiln.
It is a further object of this invention to provide a process for the manufacture of cement clinker in a conventional long wet or dry rotary kiln characterized by reduced fuel costs per unit of product.
It is still a further object of this invention to provide a process for the manufacture of cement clinker in a conventional long wet or dry rotary kiln which permits the utilization of high volatile, solid fuels including low grade fuels in an environmentally sound manner.
It is still a further object of this invention to provide a process for the manufacture of cement clinker in a conventional long wet or dry rotary kiln characterized by reduced NOx emissions.
It is yet a further object of this invention to provide a process for the manufacture of cement clinker in a rotary kiln characterized by a reduced consumption of premium conventional fuels.
It is yet a further object of this invention to provide a process for the manufacture of cement clinker in a conventional long wet or dry rotary kiln characterized by improved operating stability in the kiln.
It is yet another object of this invention to provide a process for the manufacture of cement clinker in a conventional long wet or dry rotary kiln characterized by improved manufacturing efficiency as related to the thermodynamic needs of the chemical process and the operational and mechanical needs of the process equipment.